Polyester resins derived from polycarboxylic acids and polyhydric alcohols have been widely used as resinous vehicle in coatings, because an excellent film with toughness and other desirable properties can be obtained therefrom. Irrespective of the polymerization degree, the ester bondings contained are resistive against hydrolysis, which, in turn, is believed to be one of the reasons and advantages of using this polymer as resinous component.
In coatings, and especially in ship's bottom paint, if the coated surface is full of undulations and is gradually hydrolyzed or dissolved in sea water during sailing, thereby resulting a comparatively flat bottom surface, fuel cost could be greatly reduced and furthermore, the tendency for the sea life such as acorn shells or the like to stick to the bottom of ship would be greatly reduced. Therefore hydrolyzable resinous materials have become the object of public attention in recent years. However, very unfortunately, attempts at employing such material have all ended in failure partly due to the two conflicting requirements of moderate stabilization and gradual decomposition of the material in sea water.
Polyester is, as is well known, composed of water soluble polyhydric alcohol and polycarboxylic acid which is also relatively soluble in water due to the presence of polar carboxyl groups. Therefore, if something is devised to decompose said polymer into lower molecular segments or its constitutional components, then the so-called self-polishing would be realized with this material. The inventors, having the above in mind, have made endeavors to obtain a polyester which is relatively stable in water, but is decomposed or hydrolyzed gradually under sailing conditions. Surprisingly, it was found that in the preparation of polyester by the polycondensation of polycarboxylic acid and polyhydric alcohol components, if a polyhydric alcohol having the metal ester bonding of the formula ##STR1## wherein M stands for metallic element and .OMEGA. is an integer corresponding to the valency of said metal M, is used as at least part of said polyhydric alcohol component, a number of said metal-ester bondings can be introduced into the polyester chain, and the thus obtained polyester is very stable against hydrolysis under neutral conditions, but is gradually decomposed and hydrolyzed under weak alkaline conditions, at the sites of said metal-ester bondings, thereby liberating metal ions and giving water soluble segments with carboxyl groups. Sea water and sailing will offer optimum hydrolysis conditions for this polymer. On the basis of these findings, the inventors have completed the invention.